Such protection devices are usually arranged in a central portion of the steering wheel, i.e. above the location at which the steering wheel is connected to the steering column. In this arrangement, the lining facing the driver is formed by a module cover adapted both in appearance and feel to the surroundings of the steering wheel. The latter has a foamed plastic casing surrounding a steering wheel skeleton, the surface of which body is substantially non-porous and textured like leather.
If the module cover is not made by the same source, i.e. by the manufacturer of the steering wheel, it is difficult to texture the surface of the module cover such that it is not different in appearance and feel to that of the steering wheel surroundings. These technical difficulties may be practically eliminated when foam-casing the steering wheel skeleton and producing the module covers is single-sourced, it then being easy to ensure that components directly juxtaposed in the finished vehicle are produced from the same preproduct batch.
More recently, however, automotive manufacturers are increasingly demanding speedier final-assembly which is often only attainable by employing larger preassembled assembly units, these requirements also favoring production of the steering wheel single-sourced.
When, however, protection devices including steering wheel, air bag module and module cover are to be furnished as assembly units to the final assembly line, i.e. using a steering wheel having an integrated air bag, the steering wheel can no longer be mounted, as usual, by means of a nut screwed to the steering column since the already incorporated air bag module prevents access to a nut thus arranged. Other means thus need to be found for attachment of the steering wheel.
It is furthermore to be noted that the air bag module itself is made up of components which differ and as a rule are sourced from different manufacturers. When air bag modules for steering wheels are not fabricated by the steering wheel manufacturer, these modules, as assembly units, nearly always also include the module cover since the trend is towards larger assembly units and out-sourcing the assembly time involved.
In summary it can thus be established that although the desire for larger assembly units is understandable since they permit speedier assembly, this gives rise to other problems which are a disadvantage to the automotive manufacturers since it makes them more dependent on the manufacturer of the assembly units than they would care to be. If the assembly units involved, i.e. steering wheel, air bag module and module cover can be purchased from sources independent of each other, the manufacturer is able to benefit from, among other things, price advantages which are much higher than the then additional expense involved in final assembly. However, it is often the case that this freedom of choice does not exist, because at least air bag module and module cover are offered only as an assembled unit and because these assembly units are designed compatible with each other to such a degree that splitting them up would only be possible at prohibitively high expense.